I'm in the process of learning to repair guitars with the intention of doing it for a living (I hope) then building guitars and basses someday. So far what I have found from talking to a couple of people in the business is that you can either "just do it" and learn all you can by working on junk guitars and then moving up to more advanced work or find a tech. or luthier to apprentice with.

I haven't found anyone to apprentice with yet but have already learned a lot from working on old junk guitars and a few guitars that a friend has brought to me for work.

So far so good.

Definately check out Dan Erlewine's Guitar Repair book, it is an amazing book and Dan is pretty much the Buddha of guitar repair. I am sure there are other good books out there as well as useful information on the internet but be careful what you read online... one person's idea of how to re-set a neck might not be the right way even if it worked on his or her own guitar.

If you eventually would like to get into building like I intend to or want to go beyond simple setups and tweaking, look into any information on woodworking, wood and furniture finishing and of course electronics.